docs: def by_population_density(self,
                                lower=-1,
                                upper=2 ** 31,
                                zipcode_type=ZipcodeType.Standard,
                                sort_by=SimpleZipcode.population_density.name,
                                ascending=False,
                                returns=DEFAULT_LIMIT):
          """
          Search zipcode information by population density range.
  
          `population density` is `population per square miles on land`
          """
          return self.query(
              population_density_lower=lower,
              population_density_upper=upper,
              sort_by=sort_by, zipcode_type=zipcode_type,
              ascending=ascending, returns=returns,
          )